The present invention is a transparent conducting oxide film and a method of making such a film. The film has the properties of good conductivity, stability, and extended transmissivity into the infrared. The film may be used as an optical coating for devices including, but not limited to, infrared sensors, actuators, light emitting diodes, transistors, solar cells, and flat panel displays.
Transparent conducting oxide (TCO) film materials are under development for many applications that include coatings for flat panel displays, sensors1, optical limiters and switches2, and a variety of devices that rely on the non-linear optical response of their components3. Most TCO films currently under development are n-type semi-conducting materials. Unfortunately, attendant high conductivity limits their transmissivity in the infrared. There is an increased demand for optical transparency at these wavelengths, which seeks to develop conducting oxide coatings with good optical transmission in the 1 to 12 micrometer wavelength band. In many of these existing n-type oxide-based materials, the presence of free carriers associated with enhanced conductivity, also induces long wavelength absorption thereby limiting transmission in this region. Although desirable for many applications, such as heat reflecting windows, the infrared-blocking behavior can be a detriment to the performance of sensors in the infrared and near infrared regions and for non-linear optical devices that are required to operate at longer wavelengths.
Presently disclosed are methods and composition for an infrared transparent conducting oxide optical film adhered to an infrared transparent substrate, wherein the optical film comprises a mixed transition metal oxide spinel compound of the form M11-xM2xO4/3, wherein M1 and M2 are two different transition metals and x is less than or equal to 1. M1 and M2 can be selected from any of the Group VIII elements such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt. It is advantageous for M1 and M2 to be transition metals with more than one oxidation state, preferably +2 and +3, and that they be in a spinel type crystal structure. A preferred mixed transition metal oxide spinel compound is NiCo2O4. Although still p-type in nature, films of these mixed cobalt-nickel oxide compositions exhibit higher conductivity because of the contribution of polaron hopping to the conduction mechanism. Additionally, the films can be made thin enough to provide adequate transmissivity from visible wavelengths out to the 12-micrometer infrared region. Other mixed transition metal oxide compounds also were found to exhibit high p-type conductivity and wavelength transmissivity from the visible to wavelengths at least as long as 12 micrometers. One such example is the palladium nickel oxide spinel of nominal composition PdNi2O4. Variations in the Pd:Ni ratio also were found to give conducting transmissive films. The substrate used in this invention can be any infrared transparent material substrate such as, but not limited to, sapphire, silicon, zinc selenide and quartz.
In another aspect of the present invention, the optical film comprises a mixed transition metal oxide spinel compound of the form M11-xM2xO4/3, wherein M1 and M2 are two different transition metals and x is less than or equal to 1 and further comprising Li. It is advantageous to use trace to about 10% Li. It is believed that addition of small amounts of Li to the metal oxide films of the instant invention will enhance conductivity.
In another aspect of the present invention, a method of making an infrared transparent conducting oxide film on an infrared transparent substrate is provided. This method comprises the steps of preparing a precursor solution comprising nitrates of two transition metals and a reducing agent; spin casting the precursor solution to form a film on the substrate, and inducing the transparent conducting oxide formation on the substrate by thermal treatment of the film. The reducing agent is preferably glycine but could also be malonic acid or any other soluble reducing agent that assists the combustion process. It is important for the reducing agent to completely volatilize after the thermal treatment. The thermal treatment is to be provided under conditions that achieve minimum initiation combustion temperature. The duration of the thermal treatment is dependant upon the duration of the thermal treatment. For example, if the temperature of the thermal treatment is 500xc2x0 C., the corresponding duration of the treatment is about 10 minutes. Should the temperature used be less than 500xc2x0 C., the duration of the treatment would then increase to accommodate. The principle approach to the thermal treatment is to assure that the oxide is not being decomposed while achieving the minimum initiation combustion temperature.
In another aspect of the present invention, a method of making an infrared transparent conducting oxide film on an infrared transparent substrate is provided. This method comprises the steps of preparing thinned metal targets from metal alloy by milling to fit within a magnetron sputter cathode and sputtering the film onto the substrate in an atmosphere of pure oxygen at a partial pressure.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.